Flow measuring devices equipped with a magnetically inductive flow sensor are used to measure the volume flow rate of an electrically conductive fluid flowing in a flow direction through a measuring tube of the flow sensor. For this purpose, a magnetic field of maximized density is created at the flow sensor by means of a magnetic circuit arrangement electrically connected to an exciter electronics of the flow measuring device. The field passes through the fluid within the measuring volume at least in sections perpendicularly to the direction of flow and closes on itself essentially outside of the fluid. The measuring tube is usually made of a non-ferromagnetic material, in order that the magnetic field not be unfavorably influenced during measurement.
As a result of the movement of free charge carriers of the fluid in the magnetic field, an electric field is produced in the measuring volume, on the basis of the magneto-hydrodynamic principle. The electric field extends perpendicularly to the magnetic field and perpendicularly to the flow direction of the fluid. As a result, at least two measuring electrodes arranged spaced from one another in the direction of the electric field can be connected to an evaluation electronics of the flow measuring device to measure an electric voltage induced in the fluid. This voltage is a measure for the volume flow rate. The flow sensor is constructed such that the induced electric field closes on itself outside of the fluid practically exclusively by way of the evaluation electronics connected to the measuring electrodes. The induced voltage can be sensed, for example, by galvanic measuring electrodes contacting the fluid, or by non-contacting, capacitive measuring electrodes.
For guiding and coupling the magnetic field into the measuring volume, the magnetic circuit arrangement usually includes two coil cores, which are arranged along a perimeter of the measuring tube, preferably diametrally and as mirror images, spaced from one another and each with a free terminal face. A coil arrangement connected to the exciter electronics couples the magnetic field into the coil cores in such a way that it passes through the fluid flowing between the two terminal faces at least in sections perpendicularly to the flow direction.
Because of the great mechanical stability needed for such measuring tubes, they are preferably constructed of an outer, preferably metallic, support tube of predetermined strength and breadth, which is coated internally with an electrically non-conducting, insulating material of predetermined thickness, the so-called “liner”. By way of example, each of the U.S. Pat. Nos. 6,595,069, 5,280,727, 4,679,442, 4,253,340 and 3,213,685, and the JP-Y 53-51 181 discloses a magnetically inductive flow sensor having a measuring tube insertable pressure-tightly into a pipeline. The measuring tube has a first, inlet end and a second, outlet end and includes a non-ferromagnetic support tube serving as an outer shell of the measuring tube and a tube-shaped liner seated in a lumen of the support tube. The liner constrains a flowing fluid and isolates it from the support tube.
The liner serves to chemically isolate the support tube from the fluid. In the case of support tubes of high electrical conductivity, especially in the case of metallic support tubes, the liner also serves an electrical insulation between the support tube and the fluid, in order to prevent a short circuiting of the electrical field through the support tube. Appropriate design of the support tube thus enables a matching of the strength of the measuring tube to the mechanical loads present in the respective application, while the liner enables the measuring tube to meet the chemical, especially hygienic, requirements of such application. Manufacture of the liner is often done by injection molding or transfer molding. It is, however, also common to insert a completely prefabricated liner into the support tube. In JP-A 59-137 822, a method is disclosed, in which the liner is formed by softening plastic film. Usually, open-pore, especially metallic, support bodies are embedded in the liner of, most often, thermoplastic or thermosetting plastic, to lend stability to the liner, this being shown, for example, in EP-A 36 513, EP-A 581 017, JP-Y 53-51 181, JP-A 59-137 822 and the U.S. Pat. Nos. 6,595,069, 5,664,315, 5,280,727 and 4,329,879. The support body serves to stabilize the liner mechanically, especially against pressure changes and thermally-related volume changes. By way of example, U.S. Pat. No. 5,664,315 discloses a method for manufacturing a measuring tube of a magnetically inductive flow sensor having an internal liner, in which method a pre-fabricated support body in the form of an expanded-metal screen for mechanically stabilizing the liner is placed in the support tube before the insertion of the liner. Additionally, JP-Y 53-51 181 shows a tube-shaped support body having holes in its lateral surfaces, while EP-A 581 017 and U.S. Pat. No. 6,595,069 disclose sintered support bodies. The support bodies are always installed in the lumen of the measuring tube and aligned therewith, as well as being completely enclosed by insulating material, at least on the inner side toward the fluid.
Especially the flow measuring device disclosed in U.S. Pat. No. 4,253,340 includes, additionally, a support frame for holding the measuring tube and for holding an electronics housing mechanically connected with the flow sensor. The housing serves to accommodate the above-mentioned exciter and evaluation electronics near the flow sensor and to protect such, to a wide degree, from environmental influences. Measuring tube and support frame are, in this connection, only attached to one another at the inlet and outlet ends, in each case over a relatively narrow connection area. Flow measuring devices along the lines of U.S. Pat. No. 4,253,340 excel in, among other things, permitting a very compact construction.
Investigations have now, however, shown that such flow measuring devices can have a tendency to form cracks in the area of the connection between the support tube and the support frame, especially in the area of weld connections, particularly in applications in the foods and pharmaceutical industries. It was additionally determined, that this behavior can especially be traced to high and rapidly changing axial stresses in the measuring tube because of rapid temperature changes over a very wide temperature range of about 150 K (Kelvin), such as can e.g. occur during cleaning and/or sterilization of the flow meter with hot fluids. Because of the relatively high stiffness of the measuring tube and support frame, these stresses tend to be relieved particularly in the area of the weld connections, which leads to the undesired cracks at these locations.